1. Field of the Invention
The present invention relates to a control apparatus of a transistor motor. More specifically, the present invention relates to a control apparatus of a transistor motor for performing a smooth rotation by suppressing a torque ripple.
2. Description of the Prior Art
In the case of a transistor motor for use in audio equipment, video tape recorder or the like and having a permanent magnet as a magnetic field source of a rotor, the performance of such product is influenced by vibration and noise incidental to rotation of the rotor and among others a rotation irregularity is a most important factor. Since a rotation irregularity, i.e. a torque ripple, occurring when a rotational force is transmitted to a permanent magnet of a rotor as a function of a current flowing through an armature coil provided on a stator is a largest factor giving rise to a rotation irregularity, it is imperative to mitigate a torque ripple in this type of motor.
Conventionally an approach was employed in such transistor motor in which a hall-effect device is employed as a rotor position detecting means and control is made such that the output of the hall-effect device is as such power amplified to be applied to an armature coil or a current proportional to the output of the hall-effect device is caused to flow through an armature coil. This approach is referred to as a linear type and FIG. 1 shows one example of a control circuit for employing such type. Referring to FIG. 1, the outputs from hall-effect devices .alpha., .beta. and .gamma. are linearly amplified by operational amplifiers P1 to P3 and transistors Q1 to Q6, respectively, so that the outputs are applied to armature coils u.sub.1, u.sub.2, v.sub.1, v.sub.2, w.sub.1 and w.sub.2 of a transistor motor. According to this approach, a first formula of sin.sup.2 .theta.+cos.sup.2 .theta.=1 is utilized in case of the two-phase and a second formula of sin.sup.2 .theta.+sin.sup.2 (.theta.-120.degree.)+sin.sup.2 (.theta.-240.degree.)=3/2 is utilized in the case of the three-phase, whereupon the motor is driven so that no torque ripple occurs in principle. In such case, the hall-effect devices of position sensors used as control elements of the same number as that of the phase are provided for the purpose of control; however, it is usual that there is a difference between the amplitudes of the output voltages of the respective hall-effect devices due to diversification of the quality of such products even when the input conditions of such hall-effect devices are the same. Thus a torque ripple occurs unless such amplitude difference is corrected. Accordingly, this conventional approach requires a gain adjusting means for equalizing the levels of the respective phases as to the outputs of the hall-effect devices or the voltages applied to the armature coils. The FIG. 1 example employs variable resistors VR1 and VR2 as a gain adjusting means.
Another control approach often employed is the one in which the output of the position sensors are used after conversion to logical signals. For example, in the case of a three-phase motor, a so-called 120.degree. conduction control employed in which a constant current is caused to flow normally in series in the armature coils of two-phases out of three-phases.
FIG. 2 is a diagram showing one example of a control circuit employing a 120.degree. conduction control method and FIG. 3 is a graph showing waveforms of the signals at various portions in the FIG. 2 diagram. A current IU flowing through the armature coils u.sub.1 and u.sub.2, a current IV flowing through the armature coils v.sub.1 and v.sub.2 and a current IW flowing through the armature coils w.sub.1 and w.sub.2 assume waveforms of conduction and interruption at every 120.degree. in terms of the electrical angle due to on/off control of the respective transistors Q34 to Q36 and Q25 to Q27.
However, control is made such that the armature current IA being a total of two out of currents IU, IV and IW may be normally constant. At that time, the voltage V.sub.UO between the terminals U and O and the voltage V.sub.UV between the terminals U and V are each a sum of a counterelectromotive force CEF of the armature and the voltage drop V.sub.DP due to the resistance of the armature coils. According to the above described 120.degree. conduction control method, in principle approximately a 13% torque ripple is involved on the occasion of a normal state in which a sinusoidal counterelectromotive force is induced in the armature coil and the same cannot be neglected although practically it is of a less problem. Furthermore, since conduction control of the armature coils is on/off control, it is necessary to provide a filter including a relatively large capacitor to voltage applying terminals of the armature coils of the motor. Referring to FIG. 2, such filter comprises a capacitor C and a resistor R. Furthermore, a slit-like instantaneous interruption of a current is likely to occur, which entails a disadvantage that a vibration and noise likely occur.
It has also been desired that a control apparatus of a transistor motor is provided in which adjustment of the outputs of the hall-effect devices or the voltages applied to the armature coils for each motor is not required and a slit-like instantaneous interruption of a current in the armature coils is suppressed without employing the above described filter.